[U-Boot] there is no need for uboot to use linux's mtd driver!

[liu.yaojin at zte.com.cn]

i am porting uboot to our board.and i am a fresh man to the uboot.
after several days on studying uboot.i found uboot's mtd drvier framework 
for nand is very foolish!

i think uboot should just have some api,such as 
read/write_page,oob,crc,but let them be null functions.
!!!because!!! the semiconductor company's engineers will provide the 
functions!!!.so when they port uboot to their cpu ,it will very 
easy.furthermore, using the null api is good to customer and kinds of 
engineers!~

we shouldn't spend so much time on bootloader.but the author seems to make 
our work harder!

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